There are various kinds of basils, and their flavors and colors are also various. As herbs have become popular in Japan, the cultivation of basils, having green color in general, such as sweet basil in vegetable gardens has recently become popular.
Because anthocyanin dye contained in dark opal basil leaves easily decolors under alkaline and neutral conditions, fresh or freshly-picked leaves of the dark opal basil are preserved by immersing them in acetic acid. When a large amount of fresh dark opal basil leaves are to be preserved in the form immersed in acetic acid for a long time, a part of the leaves is occasionally brought into contact with air and such leaves become moldy. Therefore, only a small amount of the dark opal basil leaves was immersed in a vinegar. For example, conventionally, several sheets of the dark opal basil leaves were used in the form immersed in a vinegar in a bottle or, when a large amount of the leaves are immersed in the vinegar, the leaves must be taken out of the vinegar after the storage of the immersed leaves for a given period of time but before the molds are formed.
Under these circumstances, dark opal basil leaves are used only for producing the extract thereof because the amount of the leaves which can be immersed in vinegar is only small. In such a case, the dye of the leaves is extracted in acetic acid used in a relatively large amount and, as a result, the leaves have a reduced dye content. As a result, the dark opal basil leaves immersed in a small amount of vinegar cannot be considered to be sufficient for the dressing for meats and vegetables.
On the other hand, when the leaves are used in a large amount, the immersion thereof must be finished in a short period of time before the molding. In such a case, the extract having a high dye concentration cannot be easily obtained because the extraction of the dye from the dark opal basil leaves is only weak.